Chilcot said then-Prime Minister Tony Blair's government presented an assessment of the threat posed by Saddam Hussein's weapons with "certainty that was not justified"

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In this Thursday Dec. 22, 2005, file photo, the then Britain's Prime Minister Tony Blair meets soldiers at Shaibah logistics base, Basra, Iraq. Thirteen years after British troops marched into Iraq and seven years after they left a country that's still mired in violence, a mammoth official report will be published Wednesday July 6, 2016.

The head of Britain's Iraq War inquiry released a damning report Wednesday on a conflict he says was mounted on flawed intelligence, was executed with "wholly inadequate" planning, and ended "a long way from success."

Retired civil servant John Chilcot, who oversaw the seven-year inquiry, said "the U.K. chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted. Military action at that time was not a last resort."

The 2.6-million-word report is an exhaustive verdict on a divisive conflict that — by the time British combat forces left in 2009 — had killed 179 British troops, almost 4,500 American personnel and more than 100,000 Iraqis.

Chilcot said then-Prime Minister Tony Blair's government presented an assessment of the threat posed by Saddam Hussein's weapons with "certainty that was not justified." He also found military planning for the war and its aftermath were not up to the task.

"The people of Iraq have suffered greatly" because of a military intervention "which went badly wrong," he said. But he refrained from saying whether the 2003 invasion was legal, and did not find that Blair and his government knowingly misled Parliament or the British public.

Chilcot heard from 150 witnesses and analyzed 150,000 documents. His conclusions are a blow to Blair, who told President George W. Bush eight months before the March 2003 invasion — without consulting government colleagues — "I will be with you whatever."

The report says Blair went to war to stand shoulder-to-shoulder with Britain's main ally, only to find the U.K. excluded from most important decision-making about the military campaign and its aftermath.

The report concludes that Britain's combat mission, which ended in 2009, did not achieve the objectives laid out in 2003 and saw British forces make a "humiliating" deal with militias in southern Iraq to avoid attacks.

"The U.K. failed to plan or prepare for the major reconstruction program required in Iraq," the report said.

The war has overshadowed the legacy of Blair, whose government has been accused of exaggerating intelligence about Saddam's alleged weapons of mass destruction in order to build support for the invasion.

Chilcot criticized spy chiefs who failed to ensure their partial intelligence about Saddam's weapons was not hardened into certainty by government spin. He said they also failed to consider "that Iraq might no longer have chemical biological or nuclear weapons" — which turned out to be the case.

The report said the widespread perception that the government had exaggerated intelligence evidence "has produced a damaging legacy, including undermining trust and confidence in government statements."

Blair — who declined to comment on the report before publication — has always said his government did not invent or distort intelligence.

The report also faults him for making key decisions with only a few key aides rather than through collective Cabinet consultation.

Chilcot's report has been repeatedly delayed, in part by wrangling over the inclusion of classified material, including conversations between Blair and Bush. Some of Blair's pre-war letters to the president are published in Chilcot's report, but not Bush's replies.

Anti-war activists and relatives of some dead British troops hoped the report would find the conflict illegal, opening the way for Blair to be prosecuted for war crimes.